This invention relates to magnetorheological dampers and, in particular, to surface finishes for sliding wear surfaces of the damper body in magnetorheological dampers.
Magnetorheological (MR) dampening devices or dampers are used in various automotive applications to dampen and control vibration, such as in suspension systems and engine mounts for vehicles, and in other applications for fluid-assisted actuation, such as flow-control valves, brake assemblies, and clutch assemblies. The MR damper device generally comprises a piston mounted within a damper body and adapted for sliding motion in sliding contact with the interior of the damper body. The damper body is typically formed of a plain carbon steel, which commonly has a hardness less than about 240 Brinell hardness number (BHN), and contains a volume of a magnetorheological (MR) fluid. The operation of the MR damper relies upon the unique properties of MR fluids, which are fluid compositions that undergo a change in apparent viscosity in the presence of a magnetic field.
The MR fluid includes numerous microparticles of a ferromagnetic material suspended in a low-viscosity carrier liquid. The microparticles have a size distribution ranging from about 1 xcexcm to about 25 xcexcm and are preferably present in an amount between about 50 wt. % and about 90 wt. % of the total composition of the MR fluid. In the presence of an applied magnetic field, the microparticles become polarized and organize into agglomerations or chains, which increases the apparent viscosity or flow resistance of the fluid. When the applied magnetic field is removed, the microparticles return to an unorganized or random state and the viscosity of the MR fluid is lowered.
Hydraulic cylinders include a tubular body, typically formed of a plain carbon steel, and a piston moving within the interior of the tubular body. The piston usually includes a soft piston band that provides a seal between the piston and the tubular body. The piston band readily incorporates any microparticles that are either intentionally present in the cylinder fluid or present as a contaminant in the hydraulic fluid. The wear resistance of the hydraulic cylinder may be improved by coating the interior of the tubular body with a hard chromium plating having a nominal thickness of about 5 xcexcm. This hard chromium layer provides a sliding wear surface for the piston band and a sliding wear surface for any other moving component that slidingly contacts the interior of the tubular body.
Plain carbon steel is known to experience xe2x80x9chigh-stress abrasionxe2x80x9d during a stroke of the piston when microparticles penetrate through the sliding wear surface afforded by the hard chromium plating and are forced into the underlying plain carbon steel. High-stress abrasion is characterized by plastic deformation that creates or plows deep scratches in the layer of hard chromium plating and the plain carbon steel. The surface damage associated with high-stress abrasion is more severe and the rate of material removal or wear rate is significantly more rapid than for a competing wear mode known as xe2x80x9clow-stress abrasion.xe2x80x9d In the more desirable low-stress abrasion wear mode, microparticles do not penetrate through the layer of hard chromium plating and, instead, cut shallow furrows that remove the sliding wear surface at a relatively slow rate.
Under conditions of high-stress abrasion, the sliding wear surface of the tubular body can rapidly deteriorate due to the presence of microparticles. Once the sliding wear surface has been penetrated, high stress abrasion works to erode the underlying base material of the tubular body. As the sliding wear surface and base material erodes, the hydraulic cylinder may experience a partial or complete loss of functional capability. For example, the abrasive wear can increase the inner diameter of the tubular body near the portion of the sliding wear surface contacted by the piston. The increased diameter dramatically reduces the available dampening forces. In particular, the greatest loss in dampening force occurs for portions of the sliding wear surface near mid-stroke of the piston, where the piston has the most frequent residence during movement. Accordingly, thin layers of hard chromium plating in the 5 xcexcm range do not offer significant resistance to penetration and the wear mode remains predominantly high-stress abrasion.
The MR fluid used in MR dampers provides an ample supply of microparticles that accelerate the wear of the damper body caused by the sliding movement of the piston therein. Microparticles become readily trapped between the piston and the sliding wear surface of the damper body. High-stress abrasive wear rapidly erodes the sliding wear surface of the damper body and, as a result, conventional MR dampers are prone to premature and rapid failure at any of the various portions of the sliding wear surface. Despite considerable efforts, manufacturers of conventional MR dampers have been unable to significantly prolong the service life of the damper body. Although the application of a thin 5 xcexcm layer of a hard chromium coating would retard the failure rate of damper bodies, such thin layers would not prevent the occurrence of high-stress abrasive wear. Thus, the service life of conventional MR dampers is significantly shortened by the deficient wear resistance of conventional damper bodies.
Thus, there is a need to prevent or retard the damage to the sliding wear surfaces of the damper body of an MR damper arising from high-stress abrasive wear.
The present invention provides damper bodies for a magnetorheological (MR) damper and methods of improving the wear resistance of a damper body for an MR damper. Specifically, the wear resistance is improved by applying a thickness of hard chromium plating or chromium, greater than or equal to a minimum thickness, to the sliding wear surface of the damper body such that the wear mechanism is predominantly low-stress abrasive wear and the base material of the damper body does not experience significant high-stress abrasive wear over the vast majority of the service life of the magnetorheological damper.
According to an embodiment of the present invention, the magnetorheological damper generally comprises a damper body having a cylindrical inner surface and a piston mounted for sliding movement within the damper body. The damper body is formed of a steel and an abrasion-resistant layer comprising chromium is applied to the cylindrical inner surface. The abrasion-resistant layer, provides a sliding wear surface for sliding movement of the piston. To ensure that the low carbon steel forming the damper body does not experience significant high-stress abrasive wear over the vast majority of the service life of the magnetorheological damper, the minimum thickness of the abrasion-resistant layer is greater than or equal to about 8 xcexcm and depends upon the Brinell hardness of the steel.
In more specific embodiments, the magnetorheological damper further comprises a sealed interior space which is substantially filled with a magnetorheological fluid and divided into two chambers by the piston. The magnetorheological fluid comprises microparticles formed of a material exhibiting magnetorheological activity in the presence of a magnetic field and having a size distribution ranging between about 1 xcexcm and about 25 xcexcm. The piston includes a flow passageway that permits the magnetorheological fluid to flow between the two chambers. An electromagnetic is configured and positioned to selectively apply a magnetic field to the magnetorheological fluid flowing through the flow passageway for increasing the effective viscosity thereof.
According to another embodiment of the present invention, the magnetorheological damper comprises a damper body having a cylindrical inner surface and a piston mounted for sliding movement within the damper body. The damper body is formed of a base material having a first Brinell hardness number of at least about 90 BHN. The cylindrical inner surface is covered by a layer of a hard coating material having a thickness greater than or equal to 25 xcexcm and a second Brinell hardness, which is greater than the first Brinell hardness number of the base material. The effective hardness of the damper body is a composite of the first and second Brinell hardnesses of the base material comprising the damper body and the layer of hard coating material, respectively. An abrasion-resistant layer comprising chromium covers the layer of hard coating material and defines a sliding wear surface for sliding movement of the piston. The abrasion-resistant layer has a sufficient additional thickness greater than or equal to a minimum thickness of about 8 xcexcm such that the base material does not experience significant high-stress abrasive wear over an expected service life of the magnetorheological damper. The minimum thickness of the abrasion-resistant layer of chromium is chosen in direct relation to the effective hardness.